traffic-selector: Avoid out-of-bound array access when calculating range

traffic-selector: Avoid out-of-bound array access when calculating range

This happens for `/0` subnet masks.  In practice, it's not an issue because
if `bytes` is 0, then so are `netbits`, `bits` and `mask`.  So the two
incorrectly addressed array elements are not actually modified.  The first
operation is a `&= 0xff` and the second a `|= 0`, so nothing changes.
But some tools might not consider the values and report this as undefined
behavior, which it technically is.